Tonometers for measuring IOP were originally developed as “contact” type instruments, meaning that a portion of the instrument is brought into contact with the cornea during the measurement procedure. A well-known instrument of this type is the Goldmann applanation tonometer (GAT) originally developed during the 1950s. The GAT measures the force required to flatten (“applanate”) a known area of the cornea, and is used today as a standard against which other types of tonometers are compared to assess measurement accuracy.
Patient discomfort caused by contact tonometers such as the GAT led to the development of “non-contact” tonometers (NCTs) which operate by directing an air pulse at the patient's cornea to cause applanation. As the cornea is deformed by the fluid pulse, an optoelectronic system monitors the cornea by detecting corneally reflected light from a beam obliquely incident upon the cornea, and a peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat.
In state of the art NCTs, a pressure transducer measures the pump plenum pressure as the pulse is generated to provide a plenum pressure signal, whereby the plenum pressure at the moment applanation is achieved (indicated by a sharp peak in the applanation signal) can be determined. The plenum pressure at applanation is then converted to an IOP value in units of millimeters mercury (mmHg) using a linear regression equation stored during instrument clinical calibration relative to GAT as a reference. A primary index of an NCT's reliability is the standard deviation of differences Sd of matched pairs of NCT and GAT clinical readings.
Current NCTs provide reasonably reliable IOP measurements, however recent studies indicate that corneal effects can have a significant impact on conventional NCT readings. This is not surprising, given that the cornea must be acted upon during the pressure measurement process and the air pulse must expend some of its energy “bending” the corneal tissue itself.
During a non-contact IOP measurement, the cornea is deformed from its original convex state through a first state of applanation to a slightly concave state, and is allowed to return from concavity through a second state of applanation to convexity as the air pulse decays. Indeed, a second peak corresponding to the second state of applanation is known to occur in the applanation signal. Thus, a first plenum pressure P1 coinciding with the first or inward applanation and a second plenum pressure P2 coinciding with the second or outward applanation are available from a single deformation cycle. U.S. Pat. No. 6,419,631 describes a non-contact tonometry method in which both P1 and P2 are used to calculate IOP.
The pair of pressures P1 and P2 have not been used solely for measuring IOP, but have also been evaluated in connection with measuring intrinsic properties of the cornea that are independent of IOP. U.S. Pat. No. 6,817,981 describes “corneal hysteresis” in the dynamic system, wherein the corneal hysteresis (CH) is defined as the pressure difference between the inward applanation pressure P1 and the outward applanation pressure P2. The corneal hysteresis is used as a second parameter that is evaluated in conjunction with reported IOP to assess the degree to which the reported IOP departs from an expected norm based on clinical data.
U.S. Patent Application Publication No. 2004-0183998 A1 describes a method for determining biomechanical characteristics of corneal tissue by evaluating corneal hysteresis in conjunction with a measurable geometric parameter of the cornea, for example central corneal thickness. The method is proposed as a LASIK screening tool.
While recent attention on corneal hysteresis has contributed valuable insight, it appears that corneal hysteresis provides an incomplete characterization of the cornea's biomechanical state. This is apparent from clinical data showing statistical correlation of corneal hysteresis with reported IOP and change in corneal hysteresis corresponding to induced change in IOP, both of which demonstrate that corneal hysteresis is not independent of reported IOP. Furthermore, clinical data show poor to moderate correlation of corneal hysteresis with central corneal thickness, whereas a more complete indicator of corneal properties should produce a stronger correlation with central corneal thickness.